The present invention relates to an image transfer device for a copier, printer, facsimile apparatus or similar image forming apparatus.
In an apparatus of the type described, an image transfer device transfers a toner image formed on a photoconductive element, or image carrier, to a sheet or similar transfer medium. A predominant type of image transfer device has a corona discharger and transfers a toner image from a photoconductive element to the front of a transfer medium by effecting corona discharge at the rear of the medium. In addition to such a corona type device, a contact type image transfer device has been proposed which holds an electrode in contact with a transfer belt and applies a charge to the belt from a power source via the electrode to thereby transfer a toner image from a photoconductive element to a transfer medium carried on the belt or to the belt.
Regarding the contact type image transfer device, Japanese Patent Laid-Open Publication No. 231274/1991 proposes to control the output current from the transfer belt to the photoconductive element by detecting a current fed back to the belt. Japanese Patent Laid-Open Publication No. 83762/1988 uses a transfer belt whose specific volume resistance is 10.sup.10 -10.sup.13 .OMEGA. and causes the belt to hold a charge at an image transfer position and causes a conductor to discharge the belt at a medium separating position. Further, Japanese Patent Laid-Open Publication No. 96838/1978 uses a transfer belt whose specific volume resistance is 10.sup.8 -10.sup.13 .OMEGA. and discharges the belt in between adjoining photoconductive drums playing the role of color toner image carriers.
The contact type image transfer device reduces ozone and required power source voltage, compared to the corona type device. However, the problem with the transfer belt is that an adequate voltage to be applied from the power source to the belt changes due to irregularity in the resistance of the belt and changes in environment, transfer medium type, area of the toner image and so forth, obstructing adequate image transfer. Specifically, the amount of charge to deposit on the belt in response to the voltage from the power source deviates from a value necessary for adequate image transfer due to irregularity in the resistance of the belt particular to the production line, changes in the resistance ascribable to the varying environment, changes in the material and thickness of the recording medium, etc. Among them, the changes in the resistance of the belt ascribable to the production line and environment may be reduced by elaborating the production line. This, however, severely restricts the specifications and, therefore, reduces the yield and increases the cost of the belt. It follows that an image transfer device desirably operable despite some irregularity in the resistance of the belt is needed. Particularly, when the resistance of the belt is lowered, a greater current flows to ground via a drive roller and other rollers contacting the belt than to the drum via the electrodes, obstructing desirable image transfer.